The present invention relates to a focus error detector and an optical head using the same and, more particularly, to an optical head applicable to a video disk, digital audio disk (compact disk), and optical disk usable as an external storage of a computer, as well as a focus error detector for applying focusing servo to such an optical head.
An optical head for use with a video disk, digital audio disk, optical disk associated with a computer as an external storage, and other optical disks may be constructed as shown in FIG. 1 by way of example. The optical head comprises a semiconductor laser 16 for emitting a laser beam 17, a collimating lens 18 for changing the laser beam 17 to a collimated beam, a polarizing beam splitter 19, a quarter-wave plate 20, and an objective 21 for converging the beam onto an optical disk surface 10. The optical head further comprises focusing error detector means and tracking error detector means. The focusing error detector means is made up of a critical angle prism 22, and four photodetectors 12-15. The prism 22 is arranged such that light reflected from the disk surface 10 and then turned by the objective 21 to a collimated beam becomes incident to the prism 22 via the beam splitter 19 at a critical angle relative to the reflecting surface of the prism 22. When the disk surface 10 is displaced relative to the focus 11 of the lens 21, the angle of incidence to the prism 22 and, therefore, the quantity of light incident to each of the photodetectors 12-15 is changed allowing the focusing error to be detected.
The problem with the focusing error detector included in the above-described optical head is that the prism which is a critical angle prism needs to have its three surfaces ground, i.e., incidence surface, reflection surface and emission surface, at the sacrifice of mass productivity and costs. An optical head using such a focusing error detector is naturally expensive.